<let/items=[1,2,3]/>
<button onClick() { items = items.length ? items.slice(0, -1) : [1,2,3] }>Toggle</button>

<div/el/>
<const/write(msg) { el().innerHTML += '\n' + msg } />

<for|outerItem| of=items>
  <div>
    <child write=write name=`${outerItem}` />
    <for|middleItem| of=items>
      <div>
        <child write=write name=`${outerItem}.${middleItem}` />
        <!-- <for|innerItem| of=items>
          <child write=write name=`${outerItem}.${middleItem}.${innerItem}` />
        </for> -->
      </div>
    </for>
  </div>
</for>